The subject invention is directed to a sheet product which is useful as separators in batteries.
Storage batteries have at least one pair of electrodes of opposite polarity and, generally, have a series of adjacent electrodes of alternating polarity. The current flow between these electrodes is maintained by an electrolyte which can be acid, alkaline or substantially neutral depending on the nature of the battery system. Alkaline batteries have found favor recently because of their high energy density and ability to form secondary rechargable battery systems.
Separators are located in the batteries between adjacent electrodes of opposite polarity to prevent direct contact between the oppositely charged electrode plates while freely permitting electrolytic conduction. Contact between plates may be due to imperfections in the plate structure or due to warping or wrinkling of the plate during use. Contact may also be due to the formation of dendrites or localized needlelike growths which tend to form on the electrodes, such as zinc dendrites in an alkaline nickel-zinc battery system. The ability to produce a separator membrane which can effectively act as a dendristatic diaphragm and inhibits shape change of the electrodes is a required criteria for forming an effective battery system.
It is generally agreed that separators should be (a) thin and light weight to aid in providing a battery of high energy density, (b) resistant to degradation and instability with respect to the battery components with which it is in contact, (c) capable of exhibiting a high degree of electrolytic conductivity (low electrolytic resistance) and (d), in battery systems where appropriate, capable of inhibiting formation and growth of dendrites and electrode shape change. The first two elements and the last two elements are each thought to be counter productive with respect to each other. For example, very thin sheets have a high surface area to volume ratio and are, therefore, more susceptible to attack by the other battery components (i.e. electrolyte) and by oxidation. With respect to the latter two criterias, it is known that separator membranes which are nonporous normally exhibit a high degree of inhibition to dendrite formation while having only low degrees of electrolytic conductivity. Microporous separators, that is those with discrete pores, usually have a high degree of electrolytic conductivity but are not able to inhibit dendrite shorting. Finally, membrane and microporous separators presently used are not capable of inhibiting electrode shape changes which normally occur during usage, especially in recycling of rechargeable systems.